Electrophotographic photosensitive layer

ABSTRACT

AN INORGANIC PHOTOCONDUCTOR IS ADDED TO A BINDER WHICH COMPRISES A COPOLYMER OF A VINYL MONOMER AND A COMPONENT EXPRESSED BY THE FOLLOWING GENERAL FORMULA   CH2=C(-R)-(Z)N-CH2-OH   WHERE R REPRESENTS H OR CH3, Z REPRESENTS COO(CH2)M, CONHCH2 OR OCH2, M REPRESENTS AN INTEGER OF FROM 1 TO 5, AND N REPRESENTS 0 OR 1, AN ALKYD RESIN COMPATIBLE WITH SAID COPOLYMER AND A POLYSIOCYANATE COMPOUND CAPABLE OF CROSSLINKING AT LEAST A PART OF SAID COPOLYMER.

May 8, 1973 SATORU HONJO ETAL 3,732,096

ELECTROPHOTOGRAPHIC PHOTOSENSITIVE LAYER Filed Jan. 8, 1971 FIG. I-

a s A L 3 3 -2ooi w 2 E a2 53 i i 0 I 2 3 (MIN) FIG.2

g F- E 8 -200- INVENTORS.

SATORU HONJYO MASAAKI TAKIMOTO ATTORNEYS,

United States Patent Ofifice 45/ 2,589 Int. Cl. G03g 5/00, 5/08 US. CI. 96-15 15 Claims ABSTRACT OF THE DISCLOSURE An inorganic photoconductor is added to a binder which comprises a copolymer of a vinyl monomer and a component expressed by the following general formula where R represents H or CH Z represents COO(CH CONHCH or OCH m represents an integer of from 1 to 5, and n represents 0 or 1, an alkyd resin compatible with said copolymer and a polyisocyanate compound capable of crosslinking at least a part of said copolymer.

BRIEF EXPLANATION OF DRAWINGS FIG. 1 and FIG. 2 are time-potential curves showing the characteristics of electrophotographic photosensitive layer according to this invention.

DETAILED DESCRIPTION OF INVENTION This invention relates to a novel composition of electrophotographic photosensitive layer, and particularly to an electrophotographic photosensitive layer containing a resin composition hardened so as to be hardly soluble in most organic solvents.

In the most common electrophotographic process an insulative photoconductive layer provided on a support material is subjected to uniform electrostatic charging, and then subjected to imagewise exposure to dissipate the charge in the areas exposed to light thereby forming an electrostatic latent image which is successively rendered visible by means of powder provided with an appropriate electrostatic charge.

The process of obtaining a visible image is called development, and at present an image of highest image quality can be obtained by liquid developing method. This method consists of electrophoretic adhesion of electrostatically charged fine powder to said latent image from a developer consisting of said powder dispersed in a highly insulative carrier liquid not destroying said latent image. Also widely employed are dry developing methods such as cascade, magnetic, aerosol developing, etc.

The electrophotographic photosensitive material known in the prior art is composed of photoconductive insulating layer which consists of inorganic photoconductive material such as zinc oxide, cadmium sulfide, titanium dioxide, zinc sulfide, etc. dispersed in an insulating film forming material and which is provided on a flexible support material such as paper.

In case of electrophotographic photosensitive material having the appearance of relatively pale color, it is possible to fix the obtained toner image thereon without transferring to another image receiving material. In certain cases where dye sensitized zinc oxide is employed, unpleasant coloration appearing when the amount of sensitizing dye is considerably large should preferably be removed after the processing.

When it is desired to obtain continuous tone image by liquid development, particularly high density image 3,732,096 Patented May 8, 1973 having an image quality substantiall equivalent to that obtainable with conventional silver halide photography, the toner image necessarily heaps up noticeably from the photosensitive coating surface. For complete fixing of such image, therefore, it becomes necessary to apply a lacquer since a sufiiciently stable image cannot be obtained by the method, for example, in which swelling of binder of photosensitive layer takes place during the liquid development to hold the toner.

The fixing by lacquering is all the more desirable when more than two mono-chromatic images are superposed on a single photosensitive sheet to obtain a multi-color print, since the complete mixing of colors of toner images can only be achieved by effectively filling the minute pores present in the toner images by means of lacquering. Also in case of multi-color image, the toner of each primary color exhibits a lower optical density compared with black toner employed in document copying etc. and therefore is required to be used in a larger quantity, which cannot be fixed stably even if the binder in the photosensitive layer is slightly swelled during the liquid development.

Examination of the above processes reveals that the period necessary for decoloration is shortened a great deal if the use of a solvent which does not dissolve the binder but can easily penetrate into the layer is allowed during the decoloration step. On the other hand, it is difiicult to make a suitable decoloration bath if the binder is thermoplastic and soluble in the solvent. This is because the decoloration of dye is usually carried out with acid or alkali, and consequently most of desirable solvent composition includes alcohol and other solvents of high dissolving power and polar nature as acetone, which will readily dissolve most thermoplastic resins.

Furthermore though lacquering can be most easily realized by a solution of film forming resin dissolved in an organic solvent selected in consideration of various factors such as drying speed, adhesion to the photosensitive layer or the affinity thereto, it is still difficult to find an appropriate solvent composition since thermoplastic resins are soluble in most solvents.

Also already known is a non-thermoplastic binder composition for electrophotography which is crosslinked with suitable crosslinking agent to have three dimensional network structure. Such compositions include various kinds of alkyd resins, epoxyesters containing unsaturated fatty acids cross linked in the presence of catalyst such as cobalt naphthenate, manganese naphthenate or organic peroxides.

It is difficult, however, to combine such resin compositions with photoconductive material sensitized with organic dyes, since cross-linking catalysts containing heavy metal salts or organic peroxides oxidizes and decompose such sensitizing dyes, making it diflicult or impossible to obtain photosensitive layers with stable high sensitivity over a long storage period.

Thus the object of this invention is to provide a novel binder composition for electrophotographic layer free of the drawbacks mentioned above.

More specifically this invention is to provide an electrophotographic photosensitive layer comprising inorganic photoconductive powdered material and binder material which comprises containing, as said binder composition, a copolymer of vinyl monomer or butadiene and a component expressed by the following general formula:

wherein R represents H or CH Z represents COO(CH CONHCH or OCH m represents an integer of from 1 to 5, and n represents or 1, an alkyd resin compatible with said copolymer and a poly-isocyanate compound capable of cross-linking at least a part of said copolymer.

Examples of the component expressed by the above general formula are hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, allyl alcohol, hydroxyethyl methacrylamide, hydroxyethyl acrylamide, hydroxyethyl vinylether etc.

These components are important not only for reacting with polyisocyanate to form crosslinked structure but also for increasing the compatibility with alkyd resins. The hydroxyl content in the copolymer is required to be Within a range between 0.5 and 5.0 wt. percent, preferably between 0.5 and 3 wt. percent.

The vinyl monomer to be copolymerized with the hydroxyl group containing monomer expressed by the above general formula can be, for example, styrene, methylstyrene, vinyl acetate, vinyl chloride, vinylidene chloride, acrylate esters, methacrylate esters, acrylonitrile, ethylene, propylene, etc. Also butadiene can be satisfactorily employed for this purpose.

The copolymer in this invention may further contain, as a minor component, compounds containing polar groups such as acrylic acid, methacrylic acid, itaconic acid, crotonic acid etc., at an amount of not exceeding ca. of the total weight of said copolymer, preferably less than 3%.

Among the various combinations of the components of said copolymer, those particularly difficult to realize such as hydroxyethyl vinylether and styrene are excluded.

Generally in case of forming a highly insulative layer from insulative binder and inorganic photoconductive materials such as ZnO, CdS, TiO ZnS etc. the dispersibility of said inorganic photoconductive material in said binder is a factor more important than the electric resistance of binder itself. Though the vinyl copolymers mentioned above are generally highly insulative themselves, they cannot Wet satisfactorily said inorganic powdered material and therefore provide a layer of poor charge retainability when used alone, and such problems cannot be resolved completely even by crosslinking with polyisocyanate.

Since alkyd resin is used as the auxiliary dispersing agent in this invention, the vinyl copolymer is required to be compatible with alkyd resin. Empirically desired are those with long aliphatic side chain such as butyl methacrylate, ethylhexylmethacrylate, butylacrylate, or ethylhexylacrylate, and a wide compatibility can be realized by the superposed effect of containing the OH group contained in said monomer.

Thus it is desirable to use a copolymer containing at least 30% by weight, preferably ca. 80% by weight, of butyl or higher alkyl methacrylate or acrylate.

Acid radicals present in acrylic acids etc. are indispensable for realizing satisfactory afiinity with inorgainc powdered material when vinyl copolymer is exclusively employed, but in this invention they only perform secondary significance and therefore may be omitted completely.

Polyisocyanate compound can be, for example, toluene dissocyanate, hexarnethylene diisocyanate, 4,4',4-triphenylmethane, triisocyanate, diphenylmethane-4,4-diisocyanate etc., but is represented by a condensation product between 1 mol of trimethylolpropane and 3 mols of toluene diisocyanate which is scarcely volatile at room temperature, and which is commercially available from Bayer under the trade name of Desmodule L or from Japan Polyurethane Industries under the trade name of Coronate L. Also preferred is the condensation product of xylylene diisocyanate and trimethylolpropane due to its low tendency to yellowing.

As alkyd resin employable is almost any commercially available product. This may be because the OH group in the vinyl copolymer contributes to the improvement of compatibilty. Aikyd resin suitable for the present invention can be, for example, those modified .with styrene, acryl esters, drying oils, non-drying oils, phenol-formaldehyde resin, rosin etc. The alkyd resin generally contains OH groups in the molecular structure thereof except for the case of extremely long chain-length therefore crosslinked by isocyanate, but such crosslinking is not important in certain purposes. For example, alkyd resin can stand practical purposes without hardening with isocyanate in case of dry development methods or of development with liquid developer using isoparaffinic solvent as the carrier.

Among the various types of alkyd resin particularly preferred are those modified with styrene, acrylics, phenolformaldehyde or rosin.

As regards the ratio of three components mentioned above, the alkyd resin is desired to be present within a range of 2-70 wt. percent, preferably 2-40 wt. percent of the total resin components, vinyl copolymer containing OH radical is desired to be present within a range of 30-85 wt. percent thereof and polyisocyanate compound is desired to be present within a range of 25-70 wt. percent, preferably 5-60 Wt. percent.

Said polyisocyanate is observed to be already effective when present in 0.2 equivalent with respect to l equivalent of said vinyl copolymer and of OH radical contained in said alkyd resin, and can be added as high as 6-7 equivalents with respect to said OH radicals.

When the carrier liquid of liquid developer employed in the processing is provided with a relatively strong dissolving power, said polyisocyanate is recommended to be employed in an amount higher than 0.4 equivalent with respect to hydroxyl radicals contained therein. Preferable decay characteristics in various insulative liquids realized by a relatively small amount of polyisocyanate are one of the principal advantages of electrophotographic layer prepared according to this invention.

The word compatibility used in this invention can be interpreted as a considerable wide sense. According to the experiments, it is found that almost any combination free from phase separation in solution state can be satisfactorily used in this invention. It is further confirmed that such combinations as to show slight turbidity or haze in the liquid or to show opaque appearance at the film formation can also be effectively utilized for this invention so long as no definite phase separation is observed at the mixing ratio used.

Furthermore, as the complementary explanation:

(1) Alkyd resin in wide sense may be free from aromatic polybasic acid such as phthalic acid, isophthalic acid, terephthalic acid etc. However, the presence of such aromatic polybasic acid is desired when employed in this invention.

(2) Said alkyd resin may be a mixture composed of more than two species. Also it is possible to use more than two species of vinyl copolymers so long as each contains monomer unit containing OH radicals.

(3) The mixing ratio of photoconductive material to said resin may vary within a range from ca. 1:1 to ca. 20:1.

The photosensitive layer thus prepared is provided with following advantages:

(1) The photosensitive layer is suitable for reproducing continuous tone image, since this is capable of retaining surface charge for a long time even in a liquid dcveloper containing solvent with relatively high solubility parameter. Furthermore, mechanical reinforcement of toner image can be realized by dissolving a resinous material in the liquid developer and rinsing, after developing, the photosensitive layer with a weak solvent incapable of dissolving said resinous material. This method is suitable for obtaining a multi-color print by overprinting technique. Such process is disclosed in Japanese patent application No. 10,684/ 68, corresponding to U.S. patent application Ser. No. 801,179 filed Feb. 20, 1969.

(2) The photosensitive layer stands the treatment with bath for decoloring sensitizing dyes containing ketones, alcohols or esters due to its durability against various solvents. Examples of such process are disclosed in Japanese patent application No. 60,617/69, corresponding to US. patent application Ser. No. 60,117 filed July 31, 1970.

(3) This photosensitive layer enables toner image fixation with lacquering and provides prints of high image quality.

(4) This photosensitive layer withstands the atmosphere of high humidity exhibiting excellent electrophotographic characteristics in a wide range of humidity.

This invention will be further clarified by the following examples.

EXAMPLE 1 In this example employed were acryl ester modified alkyd resin varnish (non-volatiles 50%, hydroxyl value ca. 50) as alkyd resin component, vinyl copolymer containing 34% of styrene, 51% of n-butyl methacrylate, of 2-hydroxyethyl acrylate and 0.2% of acrylic acid, and Desmodule L of Bayer AG as polyisocyanate for crosslinking said binder. Four different photoconductive coatings of the formulations shown in Table 1 were applied to obtain a thickness of ca. 7 microns after drying on polyethylene terephthalate film provided with evaporated aluminum layer.

The coating dispersions were prepared using a mixed solvent composed of butyl acetate, xylene and toluene.

The dispersions containing non-volatile components shown in the table were added, as sensitizing dye, with 1.5 mg. of bromochlorophenol blue, 1.0 mg. of rose bengale and 1.3 mg. of fluorescein with respect to 10 g. of photoconductive zinc oxide. Said dyes were previously dissolved in methanol, of which amount was limited to as small amount as possible since large amount of methanol, it added, will interfere with the hardening of the resins with poly-isocyanate. For practical purpose the amount of methanol should be maintained to 510% of total amount of solvent used in the coating dispersion.

TABLE 1 Composition of Photosensitive Layer Acryl ester Measurements of electrophotographic characteristics were carried out as follows:

The dark-adapted photosensitive material was negatively charged with corona discharge, and the potential 30 seconds after the completion of charging was defined as the initial potential (A). Measurements were all carried out at a condition of 25 C. and 50-60% RH. The dark decay for 1 minute in the air was measured to give the value B. Then the surface of said material was uniformly covered with purified Decalin or isoparatfin such as Isoper H (Esso Standard Oil Co.) and further subjected to the measurement of potential. The potentials at the coating of liquid, 30 seconds thereafter and 1 minute further thereafter were defined as C, D and E respectively. Thus the decay characteristics in air, potential lowering by liquid immersion and decay characteristics in the liquid were respectively defined by B/A, C/B, and E/D. FIGS. 1 and 2 show the automatically recorded potentials respectively in case of immersion in Decalin and in Isoper H.

On the other hand photosensitivity is defined by 8/E in which E represents exposure of light source 0. I. E.;

standard A) on charged layer necessary for decreasing the surface potential thereof from the initial potential to 35% thereof. This definition is convenient since it roughly corresponds to ASA in silver halide photography. It is to be noted that the sensitivity mentioned above was corrected for the potential loss by dark decay during exposure. For example in case of an exposure of E=1 lux t seconds, the initial potential to be taken as the standard is replaced by the potential (V after dark decay for t seconds, and the actual potential at this point is to be expressed by 0.35 X V Table 2 shows the characteristic values thus obtained, of which a part is also represented in FIG. 1. Also the values A and B in said table are the average obtained from FIG. 1 and 2.

1 Unmeasurable.

This table clearly shows that the materials of this invention Nos. 2, 3, 4 are provided with far superior characteristics.

EXAMPLE 2 parts of photoconductive zinc oxide, 8.5 parts of a copolymer consisting of styrene, n-butyl methacrylate, 2-hydroxyethyl acrylate and acrylic acid (33:49:16:2) and 3.5 parts of styrene-modified alkyl resin (hydroxyl value ca. 50) were mixed in a homogenizer with an appropriate amount of butyl acetate to obtain homogeneous dispersion. The dispersion was then added with sensitizing dyes in amounts as shown in Example 1, diluted with xylene, added with 0.6 part of Desmodule L (non-volatiles 75%) of Bayer AG and applied on paper previously subjected to electroconductive treatment to obtain a thickness of 7 microns after drying. Zinc oxide was stably dispersed in the coating solution to show sufficient fluidity and coating was extremely easily carried out.

The coated sample was made to stand a day in a thermostat of 40 C., then thoroughly dark adapted and subjected to the measurements as described in Example 1. The results are shown as No. 5 in Table 2.

EXAMPLE 3 Hydroxyethyl acrylate in copolymer of the composition No. 4 in Example 1, Table 1 was replaced by hydroxyethyl methacrylate to obtain similarly satisfactory results.

EXAMPLE 4 10 parts of a copolymer consisting of 51% of n-butyl methacrylate, 34% of styrene and 15 of hydroxyethyl methacrylate, 5 parts of acryl-modified alkyd resin (hydroxyl value ca. 30) and 5 parts of a condensation product composed of 1 mol of trimethylolpropane and 3 mols of xylylene diisocyanate as polyisocyanate compound were mixed with 150 parts of zinc oxide to obtain photosensitive layer. Thus prepared was an excellent photosensitive layer with an extremely slow dark decay.

EXAMPLE 5 7 parts of a copolymer consisting of 30% of methyl methacrylate, 55% of n-butyl methacrylate and 15% of hydroxyethyl acrylamide, 7 parts of acryl-modified alkyd resin (hydroxyl value ca. 30) and 6 parts of polyisocyanate shown in Example 4 were mixed with parts of zinc oxide to obtain photoconductive layer of extremely excellent characteristics.

7 EXAMPLE 6 TABLE 3 Dark decay in Dark Deealin Initial decay in percent potential, air: B/A Sensi- No. Alkyd resin volts percent C/B E/D tivlty 6- Beckosol 1341 420 96 84 96 0. 41 7- Beckosol 1303 415 96 84 94 0. 43 8 Olester 300. -385 93 84 91 0. 52

These samples show somewhat lower sensitivity but far superior dark decay characteristics compared with No. 4 in Table 1.

What is claimed is:

1. An electrophotographic photosensitive layer comprising inorganic photoconductive powdered material and binder which comprises using a binder composition containing a copolymer of vinyl monomer and a component expressed by the following general formula:

wherein R represents H or H Z represents COO(CH CONHCH or OC H m represents an integer from 1 to 5, and n represents 0 or 1, an alkyd resin containing a hydroxyl group and compatible with said copolymer and a polyisocyanate compound capable of crosslinking at least a part of said copolymer, the hydroxyl content in said copolymer being from 0.5- by Weight.

2. A layer claimed in claim 1 wherein said alkyd resin content is 270% by weight of the whole binder.

3. A layer claimed in claim 1 wherein said component represented by the formula is a hydroxyethylmethacrylate or a hydroxyethylacrylate.

4. A layer claimed in claim 1 wherein said polyisocyanate compound is a condensation product of trimethylolpropane and toluene diisocyanate or a condensation product of trimethylolpropane and xylene diisocyanate.

5. A layer claimed in claim 1 wherein the hydroxyl content in said copolymer is from 0.5-3% by weight.

6. A layer claimed in claim 1 wherein said alkyd resin is those modified with styrene, acryl esters, phenolformaldehyde resin or rosin.

7. A layer claimed in claim 1 wherein said copolymer contains 30-80% by weight of butyl or ethylhexyl methacrylate or acrylate.

8. A layer claimed in claim 1 wherein said alkyd resin is capable of crosslinking with polyisocyanate.

9. A layer claimed in claim 1 wherein said copolymer contains 30-80% of alkyl ester of methacrylic acid or acrylic acid and said alkyl group containing 422 carbon atoms.

10. A layer claimed in claim 2 wherein said alkyd resin content is 2 to 40% by weight of the whole binder.

11. A layer claimed in claim 5 wherein the hydroxyl content in said copolymer is from 0.5 to 3% by weight.

12. A layer claimed in claim 1 wherein said copolymer is 30 to 85% by weight of the whole binder.

13. A layer claimed in claim 1 wherein said polyisocyanate compound is 2.5 to 70% by weight of the whole binder.

14. A layer claimed in claim 13 wherein said polyisocyanate compound is 5 to by weight of the whole binder.

15. A layer claimed in claim 1 wherein said vinyl monomer is butadiene.

References Cited UNITED STATES PATENTS 3,437,481 4/1969 Graver 96-1.'8 3,408,185 10/1968 Mammino 961.5 3,547,747 1/1971 Dastoor 961.8

GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R. 

